Intracellular calcium concentration is an important predictor of numerous cellular activities, which include neuronal activation, muscle cell contraction and second messenger signaling. A sensitive and convenient technique to monitor the intracellular calcium levels is through the genetically encoded calcium indicator (GECI). Among the GECIs, green fluorescent protein (GFP) based calcium sensors named GCaMPs are the most efficient and widely used tool. GCaMPs have a backbone of circularly permutated GFP protein (Baird et al, Proc. Natl. Acad. Sci. USA Vol. 96, pp. 11241-11246, 1999) fused with a chicken myosin light chain (M13) at N-terminus and a calmodulin (CaM) at C-terminus Some GCaMPs yield distinct fluorescence emission spectra (Zhao et al, 30 September Vol. 333, 2011).